1. Field of the Invention
The present invention relates to a golf club head, of which loft angle is 6 to 30 degrees and depth of the center of gravity is not less than 18 mm, and which has a ball striking face increased in the coefficient of friction at high load so as to decrease the back spin of a hit ball and thereby to increase the flying distance.
2. Description of the Related Art
For golf clubs such as driver, fairway wood or similar metal wood (hereinafter genericly called wood club) having a club head of which center of gravity is relatively deep, it is particularly important to gain a long flying distance.
It is known that the flying distance performance of a golf club depends greatly on the depth of the center of gravity and back spin of a hit ball, and the greater the gravity center depth, the higher the flying height.
In case of iron clubs of which gravity center depth is small, it is difficult to make the flying height higher in comparison with the wood clubs. Therefore, when golfers whose golf club head speed is relatively slow use long iron clubs or the like of which loft angle is less than about 30 degrees, the back spin is usually increased to obtain a sufficient trajectory height and thereby to increase the flying distance. On the other hand, golfers whose golf club head speed is fast had better decrease the back spin rate in order to suppress the trajectory height so as to obtain a maximum flying distance.
In case of the wood clubs of which gravity center depth is more than about 18 mm, a sufficient elevation angle can be obtained when striking a ball even if the loft angle is smaller than about 30 degrees. Therefore, the flying distance may be increased by decreasing the back spin.
General amateur golfers are however, liable to give an excessive back spin to a golf ball and fail to increase the flying distance. Therefore, it is necessary for the wood clubs to decrease back spin in order to obtain a longer flying distance.
In the laid-open Japanese Patent application No. 61-272067, in order to decrease back spin, the coefficient of friction of the ball striking face is decreased. It seems to be an effective method. Actually, however, in case of the wood clubs, the back spin is not decreased, and thus there is no effect on increasing the flying distance.
In the circumstances, the present inventors studied back spin of a golf ball hit by the wood clubs and discovered that the back spin is closely related with the maximum static friction coefficient at a high load of 400 kgf of the ball striking face, and that, in case of the wood clubs, back spin is decreased when the coefficient of friction becomes larger, in contrast to the conventional teaching.
A perfect explanation of this reason requires a further detailed analysis of complicated deformation behavior of a golf ball at the time of impact, but one factor was estimated to be the effect of internal spin (distortional vibration) of a core of the ball (B) occurring in a moment when struck by the ball striking face 4 of the club hread.
FIGS. 7(a) to 7(d) show struck state of a ball B in time sequence. And FIG. 8 is a graph showing the relation between the shearing force between the core B1 (position P1) and cover B2 (position P2) of the ball B, and the time elapsed from the impact.
As shown in FIG. 7(a), as the hit ball (B) is deformed, it contacts with the ball striking face 4 in a relatively wide area. The ball (B) receives a frictional force (M) in a direction along the ball striking face 4 as well as a vertical force in a normal direction to the ball striking face 4.
Hitherto, only the frictional force M was taken into consideration, and it was believed that the back spin of a ball (B) would increase as the frictional force (M) was larger.
The present inventors however discovered that as shown in FIG. 7(b), the cover B2 of a ball (B) is pulled in the direction of frictional force (M), and a distortion occurs between the cover B2 (position P2) and the core B1 (position P1), and a shearing force is produced therebetween.
Such distorted state returns to the original state as the deformed ball (B) starts to restore its original shape. During this restoring process, however, a shearing force in the reverse direction to that of the frictional force (M) occurs between the core B1 and the cover B2. And after the positions P1 and P2 return to the normal state as shown in FIG. 7(c), the returning motion continues until the positions P1 and P2 are reversed as shown in FIG. 7(d). In this state, the ball is launched. Thus, the core B1 in the hit ball (B) has an internal spin which is reverse to the back spin, and the back spin becomes lower when the internal spin is larger. The effect of such internal spin on reducing the back spin is particularly notable in the wood clubs having a loft angle of not more than 30 degrees.
It is therefore, an object of the present invention to provide a golf club head in which the ball striking face is increased in the high-load friction coefficient to decrease the back spin and thereby to increase the flying distance.
According to the present invention, a golf club head comprises a ball striking face having a loft angle of from 6 to 30 degrees, and has a depth of the gravity center of not less than 18 mm, wherein
the ball striking face is provided with a high frictional region of which maximum static friction coefficient at a high load of 400 kgf is not less than 0.35.
Preferably, the friction coefficient is not less than 0.40.